1. Field of the Invention
The present invention relates to an integrating analog-to-digital converter and, more particularly, to a highly simplified and efficient auto-polarity integrating analog-to-digital converter including auto-zeroing of offset errors.
2. Description of the Prior Art
The present invention relates to analog-to-digital converters of the type which alternately charge and discharge an energy storing means for converting an unknown input signal into a digital signal. The charging and discharging of the energy storing means is done at different rates, at least one of such rates being dependent upon the amplitude of the unknown signal. By comparing the respective times of the charge and discharge cycles, a signal proportional to the amplitude of the input signal can be derived. This signal is converted into a digital signal by counting clock pulses during the charging and/or discharging cycles.
It is desirable that such a system be capable of operating automatically in response to a bipolar input signal, i.e. one which is either positive or negative with respect to a common potential. However, in such a system, it is critical to be able to accurately locate the point of common potential between the positive and negative full scale values. It is this necessity to auto-zero which has been the subject of numerous systems, since it is known that electrical circuit components inherently drift and it is impossible to make a system which will remain stable at a given common potential.
Theoretically, in order for a system to be stable, that which occurs during the charging cycle of the energy storing means must be exactly related to that which occurs during the discharging cycle of the energy storing means. In all known prior art systems, different circuits and/or circuit elements are involved in the charging and discharging cycles. Therefore, if one of these circuits or circuit elements drift, it will affect one cycle but not the other. Prior art attempts to solve this problem have been directed towards determining the amount of the drift and compensating therefor.